i've read that's it's better to replace the 305tbi, but then what kind of engine should i get then i was thinking alone the line of an Ls1 if it will fit into an 91 firebird, or if you have any other suggestion let me know engine wise or would you recomend

 

A zz4 crate engine is a decent replacement engine that will bolt in easily. You could also find a used L98 & computer/wiring to rebuild if you want fuel injection. A LS1 will not bolt directly in, and most likely would cost more than either of the two I just mentioned, although it will put up much more power.

 

Aren't LS1s a nightmare with all the electrical components. I agree the ZZ4 is a very good choice as well.